1. Field of the Invention
This invention relates to a power transmission device for a press machine for transmitting the rotary motion of a fly-wheel.
2. Description of the Prior Art
As for one of power transmission devices for a press machine for converting the uniform rotary motion of a fly-wheel into a non-uniform rotary motion and transmitting the non-uniform rotary motion to a crankshaft, there is a device comprising a pinion rotated around a rotation axis of a fly-wheel upon receipt of the rotation of the fly-wheel, a main gear geared with the pinion, and a pair of links for transmitting the rotation of the main gear to the crankshaft (refer to, e.g., Japanese Patent Public Disclosure (KOKAI) No. 60-124498). One of the links is pivotally connected to an eccentric portion of the main gear at one end thereof, and the other link is pivotally connected to the other end of the one link and fixed to the crankshaft.
In the conventional power transmission device using gears, however, the rotation axes of both gears are separated from each other, and therefore, an unbalanced force acts between both gears, resulting in the generation of vibration. In addition, the power transmission device is larger than the dimension corresponding to the distance between the rotation axes of both gears, and, what is more, complicated.
As for the other one of the power transmission devices for a press machine for converting the uniform rotary motion of a fly-wheel into a non-uniform rotary motion and transmitting the non-uniform rotary motion to a crankshaft, there is a device comprising a rotary shaft rotated by a fly-wheel, a first arm mounted on the rotary shaft, a second arm mounted on the crankshaft, and a link pivotally connected to first and second arms at different portions (Japanese Utility Model Public Disclosure (KOKAI) No. 1-108365) issued May 2, 1991, no. 3-47686.
In the conventional power transmission device using the arms and the link, however, both arms are just pivotally connected by the link and the rotation axis of an input shaft (the rotary shaft mentioned above) cannot be substantially coincided with that of an output shaft. As a result, vibration is generated due to the eccentricity between the rotation axis of the input shaft and that of the output shaft (a shaft rotated by the second arm), and a casing for supporting the input shaft is required. Suppose the rotation axis of the input shaft is coincided with that of the output shaft in this conventional device, the uniform rotary motion of the input shaft is only transmitted to the output shaft as it is and the uniform rotary motion cannot be converted into any non-uniform rotary motion.